Headroom refers to the unused space or clearance above a person's head, object or measurement. In practical terms, it is often used in relation to vehicles such as cars and airplanes where it describes the vertical distance between the top of the vehicle and an overhead obstruction like bridges, tunnels or overpasses.
In aviation, headroom refers to the altitude above sea level that a pilot has available for maneuvering while taking off or landing. In this context, having adequate headroom means there is enough space between the aircraft and any obstruction on the ground or in the air.
The term can also be used figuratively to describe a situation where there is room for improvement or growth. For example, if a company has "headroom" for expansion it means that they have the potential to grow their business beyond its current state.